In many theoretical concepts of liquid dynamics and the glass transition the structural relaxation is considered to depend directly on the available free volume around each molecule. Typically, this molecular quantity approximated by the macroscopic density, i.e. a macroscopic average. However, particularly hydrogen-bonding liquids as well as polymers defy a proper description by these approaches (e.g. the failure of density-scaling in such materials). To investigate densification - and thus gain insight into vitrification - on the molecular scale, we carried out infrared spectroscopy measurements on a series of polyalcohols. By analyzing specific molecular vibrations and correlating them with interatomic bond lengths, the thermal expansion of intramolecular covalent bonds and strong intermolecular hydrogen bridges is quantified. Employing this method to pure water as model system of simple molecular structure (i.e. without internal conformational changes) allows us to reproduce macroscopic density from the intermolecular distances which verifies the reliability of the method. Turning to the polyalcohol samples, we find pronounced differences between intra- and intermolecular expansion which confirm the dominance of the latter. Surprisingly however, the overall thermal expansion (i.e. the cube root of inverse density) is even bigger than that of the strong hydrogen bridges. This suggests that weak hydrogen bridges dominate thermal expansion while the strong hydrogen bridges clearly control the dynamics, i.e. the structural relaxation. With this chemically specific view at expansivity, it is clear why relaxation dynamics in hydrogen-bonding materials is not properly described by the density-scaling concept. These results demonstrate that inhomogeneities on intra- and intermolecular scale can play distinct roles in densification and orientational relaxation and require a careful consideration in a comprehensive theoretical description.

When molecular heterogeneities matter: thermal expansion and relaxation time in polyalcohols

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